This invention relates in general to phase shift keyed (PSK) communication systems and specifically to carrier recovery for demodulation of PSK signals.
In recent years, cable connected television systems have become extremely popular. In such systems a "head end" is physically linked to a large number of subscriber converters/decoders by means of individual coaxial cables to provide a wide variety of television-related programming and services. It is customary for different charges to be assessed for different types of service. In general, each cable subscriber has a uniquely identifiable decoder. The head end either includes or has access to a computer which, by appropriate data signals, controls functioning of the individual subscriber units in accordance with the type of service selected, prepares bills and performs other "housekeeping" chores.
There is growing interest in so-called "pay per view" cable systems in which a subscriber is billed for any additional or specially selected programs, rather than simply subscribing to a particular class of service on a regular basis. Thus, for example, if a subscriber wanted to watch a scrambled "pay" movie, he would communicate with the head end which would respond with an appropriate signal to enable the subscriber decoder to descramble the desired channel. The signalling can, of course, be accomplished by telephone communication, but that is cumbersome, time consuming and relatively expensive.
It has been proposed to permit subscriber-to-head end communication over the cable by transmitting data on a carrier frequency that is below the normal television signal frequency range. It has further been proposed to transmit binary data grouped in "packets", each including a number of "preamble" bits to enable "lockup" of the transmitted signal, a group of address bits to associate the data with the particular subscriber unit, a group of bits for conveying the desired information from the subscriber and a small group of bits for error detection of the received data. Since the data is transmitted in binary form, that is with 1's and 0's, phase shift keying of a carrier wave is an attractive technique because a zero degrees phase can be used to indicate a "1" and the opposite one hundred eighty degrees phase used to indicate a "0".
In phase shift keying systems, there is no readily recoverable carrier, at least for all practical purposes. There are of course techniques for sending a carrier sample, but they involve an extra signal and are subject to noise problems in the system. What is needed is a simple method of deriving a carrier that is substantially "locked" to the received signal for demodulating the received signal and determining its phase changes. In this connection, the terms lockup and locked mean to have substantially the same frequency and one of the phases of the received carrier signal.
It is known in the prior art to multiply the frequency of the received binary PSK signal by a factor of two, which eliminates the phase information, filter the output and to divide the resulting double frequency wave by a factor of two to obtain a constant phase carrier of the same frequency as the incoming signal. By synchronous demodulation, the phase changes in the incoming signal may then readily be determined.
This prior art PSK carrier recovery circuit is adequate in an ideal, that is, a noise-free, environment. In the presence of noise, however, the prior art circuit is often incapable of differentiating the signal from the noise and is therefore unreliable in recovering the carrier. Eliminating the noise requires narrow band RF filtering which may not be desirable or practical. A difficulty in using a phase locked loop (PLL) with a data packet transmission cable system is that there is a lockup time associated with the PLL, as is true for any feedback type control system. To eliminate effects of noise, the PLL requires a narrow bandwidth, which increases the lockup time. Another difficulty in using PLL circuits with a cable data packet transmission system is that the frequencies of the oscillator in each subscriber unit, through crystal controlled, may vary. To compensate for these deficiencies, a long preamble, that is string of bits at the beginning of the packet, is required to assure that the recovered carrier frequency is properly tracking the incoming carrier frequency before data detection occurs. A long preamble is not only inefficient in terms of signal transmission, but it adds to system cost and increases the time required for deciphering subscriber messages. Considering that a single cable head end may need to communicate with large numbers of subscriber units, the length of the preamble becomes quite significant indeed. Thus there is a need in the art for a PSK recovery system that is effective in noisy environments, requires only a short lockup time and which uses inexpensive filtering.